1. Field of the Invention
This invention relates generally to methods and apparatus for predicting a radiation dose and more specifically to simulating the total radiation dose for a product undergoing irradiation at multiple points within an irradiation cell.
2. Description of the Related Art
Numerous manufactured goods, including medical devices and accessories, pharmaceutical or biotech manufacturing supplies, foods, etc., undergo exposure to radiation energy. This exposure is often required to reduce the counts of microorganisms and bacteria to acceptable levels or to alter the characteristics of the product or its materials. The exposure process, referred to as “sterilization” or “irradiation,” is typically achieved by exposing the product to Gamma rays, X-rays or other radiation sources for a predetermined length of time to achieve the desired result. The result is expressed as a dosage, typically in units of Kilograys or Megarads, and is measured at one or more locations on and/or within the product volume. The longer a product is exposed to the radiation, the higher its dose.
For products requiring reduced microorganism count, the location of the minimum received dose is of special significance, since it is there that the highest residual microorganism count would likely remain after exposure. At the opposite extreme, exposure to too much radiation can adversely affect the characteristics of a product. For example, certain plastics turn yellow or craze when overexposed. Manufacturers are therefore interested in the locations and amounts of the minimum and maximum dose extremes in order to specify and verify processing parameters. Determination of these extremes is difficult, time consuming and imprecise. As described below, the determination of these quantities is highly dependent on a number of variables, many of which are unknown or uncontrollable at the time any given product is irradiated. The net result is that irradiated products may not receive the prescribed dosage during routine processing. The actual delivered dose can be verified using different methods, including radiation sensitive strips called “Dosimeters.” This verification though can take place only during or after the radiation process. Accordingly, no guidance is provided for predicting or estimating doses that will be delivered to the product under any particular processing situation. Without any predictive capability, the product is at risk if the dosage delivered is outside of the minimum or maximum doses.
Since the materials used in the radioactive source, for example radioactive isotopes composed of Cobalt 60, can be expensive and dangerous to handle, the processing is done in specially built cells having thick concrete walls. The specialized knowledge, high capital costs and ongoing expenses involved in constructing, owning and operating such installations precludes most manufacturers from operating their own sterilization facilities. Instead, a number of firms provide contract sterilization services on the open market. The radioactive isotope is costly and constantly decaying (Cobalt 60 loses one-half of its potency every 5-¼ years), therefore, contract sterilization firms design their cells and schedule production runs to make efficient use of the radioactive source. Cells are typically designed to accommodate many different products simultaneously. In addition the cells are routinely arranged to fill as much of the cell volume as is practicable. Typically, a conveyor transports the product through a series of rows, arrayed on either side of the centrally located radioactive source material. The product moves through a series of predetermined positions, dwelling at each position for a predetermined period of time. In some cell configurations, products pass through the cell multiple times, each time at a different height or level.
The arrangement of products in numerous rows, sometimes on more than one level, means that a given product will clearly “see” the radioactive source only when it is in a row immediately adjacent to a source. Elsewhere, it will be partially shielded by products and other elements between it and the source. High density intervening product will absorb more of the incident energy than low density intervening product. The effect is severe enough so that under many conditions, the subject product will receive dosages outside the specified range. To avoid this unacceptable consequence, many products cannot be run with those that would adversely affect its dose. Faced with the need to fill the cell efficiently while remaining mindful of the dosage specifications of each product, cell operators employ certain heuristic rules when developing their daily production schedules. These rules typically rely on the product characteristics (for example, the product density) and characterization data taken on the subject product. The characterization data includes the collection of measured doses taken on a sample product under known conditions, in order to establish the parameters for future processing. Armed with the characterization data, product characteristics and the firm's scheduling rules, products are scheduled and selectively monitored to assure compliance.
Numerous problems result from this approach. Owing to the large number of products available to be processed at any given time, products are rarely run with the same product mix as when they were characterized. Since the surrounding product can have a substantial effect on the dose received by a given product as described above, some products are likely to exceed their processing specifications (i.e., fall outside the specified range). Moreover, since the minimum and maximum locations determined during characterization can shift when the product is processed under different conditions, a product could receive less than the specified minimum dose yet go undetected, thereby frustrating the essential purpose of sterilization. Additionally, the time and expense of placing dosimeters on products, and the logistics of handling, measuring and recording their values, makes it infeasible to exhaustively monitor every product in a production run. The diverse product mix, disparate processing specifications and delivery commitments must be reconciled into an efficient production schedule using only simple rules for inferring a resultant dose. The result is sub-optimal schedules that make poor use of the costly radioactive source, and the possibility of improperly delivered dosages. The present inability of the production scheduler to predict the effect of a production schedule on the dosages delivered to any product leads to reliance on subjective factors. Thus the achievement of consistent processing results becomes unattainable.
Public domain software such as QAD are not capable of being utilized in a production environment. These public domain methods employ classical ray tracing techniques but require individual, successive geometry definitions for every step in order to simulate the movement of a product past a radiation source. As the product advances through the predetermined positions, a new set of geometric definitions must be supplied at each position and exhaustively calculated, and the ray tracing techniques must be applied for each point at each predetermined position. The excessive amount of time required to run the ray tracing techniques, including the exhaustive geometric calculations performed at each step, and the burden of providing the input at each step as the product moves through the irradiator cell, renders the classical ray tracing techniques impractical for production use.
As a result, there is a need to solve the problems of the prior art to provide a method and apparatus for simulating the radiation dose for points on a product at each position of the product in the irradiator cell and a total radiation dose received for the points on the product through the irradiator cell.